The present invention relates to a gas turbine moving blade, and, more particularly, to a gas turbine blade having a platform undercut with improved thermal stress relief.
Gas turbine blades, also referred to as buckets, are exposed to high temperature combustion gases, and, consequently, are subject to high thermal stresses. Methods are known in the art for cooling the blades and reducing the thermal stresses. FIGS. 1-3 show one example of a prior art air-cooled moving blade. High pressure air 2, discharged from a compressor, is introduced into an interior of an air-cooled blade from a blade root bottom portion 4. The high pressure air, after cooling a shank portion 6, a platform 8 and a blade profile portion (or airfoil) 10, flows out of fine holes 12 provided at a blade face, or out of fine holes 14 provided at a blade tip portion. Also, fine holes 12 are provided at a blade trailing edge portion 13 of the blade, through which the high pressure air flows to cool the trailing edge of the blade. Thus, the high pressure air cools the metal temperature of the moving blade.
Highly cooled gas turbine buckets experience high temperature mismatches at the interface of the hot airfoil and the relatively cooler shank portion of the bucket platform. These high temperature differences produce thermal deformations at the bucket platform, which are incompatible with those of the airfoil. In the prior art, the airfoil is attached to a bucket platform that is of greater stiffness than the airfoil. When the airfoil is forced to follow the displacement of the shank and platform, high thermal stresses occur on the airfoil, particularly in the thin trailing edge region. These high thermal stresses are present during transient engine operation as well as steady state, full speed, full load conditions, and can lead to crack initiation and propagation. These cracks potentially can ultimately lead to catastrophic failure of the component.
U.S. Pat. No. 5,947,687 discloses a gas turbine moving blade (FIGS. 1-3) having a groove 16 on the trailing side 18 of the platform of a turbine blade, designed to suppress a high thermal stress at the attachment point of the airfoil trailing edge and platform that occurs during transient operating conditions, i.e., starting and stopping of the turbine. However, the groove has a depth which does not enter a stress line of the platform caused by the load on the airfoil. Since the groove does not enter a stress line, it does not affect the load path through the trailing edge of the airfoil, and the groove is, therefore, not highly stressed. Also, this groove extends along the entire length of the platform, from the concave side 20 of the blade to the convex side 24, along a circumference of the turbine, parallel to a plane of rotation of the turbine. In this configuration, the groove affects blade natural frequencies, thereby potentially inducing additional mechanical vibratory stress on the blade.
It is therefore seen to be desirable to reduce the likelihood of initiating cracks in the root trailing edge region of the airfoil by reducing the thermal and mechanical stresses that occur due to the mismatch between the airfoil and the shank.
The present invention provides a gas turbine moving blade in which a groove is introduced in the bucket platform, at an angle with respect to a mean camber line of the airfoil, such that the groove begins on the concave side of the platform and exits the platform on the trailing edge side of the bucket shank cover plate. In alternative embodiments, the cross-section of the groove may be circular, elliptical, or square with simple or compound radii, rectangular, or polygonal, in which the groove is defined by two or more planes. This groove has a depth which will enter a stress line of the platform caused by a load encountered by the blade, and will change the load path direction away from the trailing edge.
The location and depth of the groove of the present invention results in a reduced mechanical as well as thermal stress condition in the airfoil root trailing edge and a higher stressed condition in the groove. An increase in the fatigue capability of this region of the component is possible because the groove is located in a region of cooler metal temperatures having greater material fatigue strength. This groove, additionally, provides a decrease in the mechanical stress in the trailing edge by cutting into the load path of the airfoil, thus having an overall greater benefit in the fatigue life of the region.